the hierarchical layout vs. flat layout

[xworld2008]

the hierarchical vs. flat layout which is better ? And someone said when gatecounter lower than 350k, use flat layout , above 350k use hierarchical . How do you think the gatecounter of flat and hierarchical.

 

[leeenghan]

Hi,

I think always go for flat if there is a choice. Hierarchical layout always degrade the quality of the result.

Some reasons to use hierarchical layout:
1. The chip is simply to big. This has to be more than 1-2 million placeable instances with the new generation of P&R tools.
2. Some siignificant portion of the design of the chip does not change (e.g. DSP, MPEG Decorder IP) and it may be good to "harden" it into a block to save run-time when the final netlist is ready.
3. Faster turnaround time if you have many engineers/workstation/licenses, and the layout flow is established.

Regards,
Eng Han

 

[disney]

usually hierarchical layout is better, because you can use the same instance many places thus saves time/energy.... Also if you want to change changing in one will affect the rest.

Once your layout is finalized (chip level) then you always have an option to flatten, whereas vice versa is not true.

Thanks

 

[stormwolf]

If you want cost down , you may need use flat layout , it can reduce area, and flat layout wire lenght small than hierarchical, flat layout clock tree skew time is small , but flat layout time is very hard, I have read a paper from SNUG. A 1M or 5M ( i have forget) gate counter flat layout need 5 day use ASTRO.